<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      background-color: pink;
    }

    .nav {
      color: red;
    }

    .box {
      width: 300px;
      height: 300px;
      background-color: skyblue;
      margin: 100px auto;
      padding: 10px;
      border: 1px solid #000;
    }
  </style>
</head>

<body>
  <div class="nav">123</div>
  <script>
    // 1. 获取元素
    const div = document.querySelector('div')
    // 2.添加类名  class 是个关键字 我们用 className
    div.className = 'nav box'
  </script>
</body>

</html>